The synchronous buck switch-mode power converter is a commonly used topology for SMPS applications. Current sensing in this topology can be challenging and must be overcome in design. Knowing or monitoring the current being injected into the load provides protection for the power converter and can improve dynamic performance during closed loop control thereof.
Some prior technology current sensing techniques are as follows: Series sense resistor in main power path, current sense transformer, sensing voltage drop across the upper MOSFET switch, and an inductor voltage integral measurement by using an auxiliary winding to the power inductor 108. Referring now to FIG. 1, depicted is a prior technology SMPS having a series sense resistor 110 in the main power path. A voltage across the series sense resistor 110 is detected by a differential input operational amplifier 114 and a VSENSE output therefrom is proportional to the load current being supplied by the SMPS. However, the series sense resistor 110 introduces undesirable power loss. Since high efficiency is usually an overriding requirement in most SMPS applications, resistive circuit elements in the power path should be avoided or minimized. Only on rare occasions and for very specific reasons are power consuming resistances introduced into the main power control path. In auxiliary circuits, such as sequence, monitor, and control electronics of total system, high value resistors are common place, since their loss contributions are usually insignificant.
Referring to FIG. 2, depicted is a prior technology SMPS having a current sense transformer for measuring current to the load. A current sense transformer 214 has a primary connected in series with the power path of the SMPS. A sense diode 216 and sense resistor 218 provide a VSENSE output proportional to the load current being supplied by the SMPS. The current sense transformer 214 provides current monitoring for cycle-by-cycle peak current limit and peak current mode control. Power loss is minimal for this current monitoring configuration, however, implementation is expensive.
Referring to FIG. 3, depicted is a prior technology SMPS with monitoring of the on-voltage drop across the upper MOSFET switch 104. Sensing the voltage drop across the upper MOSFET switch 104 when the switch 104 is on provides a lossless signal representative of the current passing through it. A differential input operational amplifier 314 senses the voltage across the MOSFET switch 104 and produces a sense voltage output, VSENSE. However, this voltage drop is an inaccurate representation of the load current and is further subject to inaccuracies due to a high temperature coefficient of the MOSFET RDS-ON.
Referring to FIG. 4, depicted is a prior technology SMPS having an inductor voltage integral measurement by using an auxiliary winding to the power inductor. By adding an auxiliary winding 416 to the power inductor 108, a substantially lossless signal, VSENSE, representative of the current passing through the power inductor 108 is provided. However, the requirement for a coupled inductor increases the cost of the magnetic components of the SMPS.
Referring to FIG. 5, depicted is a prior technology SMPS having a matching complimentary filter for measuring current through the SMPS inductor. This matching complimentary filter is utilized in combination with the inductor coil resistance, RL, of the power inductor 108 to sense the current therethrough. The matching complimentary filter consists of a resistor 520, RF, in series with a small value capacitor 522, CF. This series connected combination is connected in parallel with the inductor 108. When the complimentary filter impedance is matched to the impedance of the power inductor 108, i.e., L/RL=RF*CF, the capacitor voltage, VCF, is directly proportional to the current through the inductor 108. This is readily shown from the following equations:VL=IL*(RL+s*L)VL=IL*RL*(1+s*(L/RL))VCF=VL/(1+s*RF*CF)VCF=IL*RL*[(1+s*(L/RL))/(1+s*RF*CF)]if L/RL=RF*CF, then VCF=IL*RL Where VL is the voltage across the inductor 108, L is the inductance in henrys of the inductor 108, RL is the coil resistance in ohms of the inductor 108, IL is the current in amperes through the inductor 108, and s is the complex frequency in the s-domain (i.e., frequency-domain). Where VCF is the voltage across the matching complimentary filter capacitor 522, CF is the capacitance in farads of the capacitor 522, and RF is the resistance in ohms of the matching complimentary filter resistor 520.
The voltage, VCF, across the capacitor 522, CF, is applied to the inputs of a differential amplifier 514 and a VSENSE output therefrom is proportional to the load current, IL, being supplied by the SMPS. Measurement of current through the inductor 108 is lossless since no resistor or impedance has been introduced into the high current path of the SMPS. However, this complimentary filter must be matched to the equivalent inductance, L, and series resistance, RL, of the inductor 108 for accurate and absolute current measurement results. This circuit also suffers from a high temperature coefficient due to the discrete component value changes over an operating temperature range, thereby reducing accuracy over the range of operating conditions of the SMPS.